A well-known adage instructs us not to "put all of our eggs into one basket." The fear, of course, is that if the basket is dropped then everything will be lost. This is usually excellent advice; but when there are compelling reasons to the contrary, it may be wiser to put our eggs in one basket--and then watch that basket carefully. Such is the case with fiber optic transmission systems whose high-capacity and economy provide compelling reasons to consolidate a staggering number of voice, video and data circuits onto a single, hair-thin glass fiber. Nevertheless, should that glass fiber fail for one reason or another, then the consequences may also be staggering. Consider the number of customers who would be inconvenienced by a fiber failure, and consider the effect when medical, transportation and emergency communication services are disrupted.
Viewed from another perspective, fiber optic transmission systems, like all communication systems, are used to communicate highly confidential information. And while it was once thought that optical systems were immune from eavesdropping, scoundrels have risen to the challenge and are now able to extract data from a "lit" fiber without breaking the transmission path. The point is simply that it is even more important in optical fiber systems to be able to detect and quickly locate any intrusion or disruption to the transmission path.
A basic fiber optic intrusion detection system is described in U.S. Pat. No. 4,904,050 issued to Dunn et al. on Feb. 27, 1990 which discloses a closed loop of optical fiber in which a beam of light is split and caused to travel around the loop in opposite directions. The loop comprises an optical cable having a large number of fibers bundled together within a common sheath. At one end of the cable, a pair of fibers are connected to an interferometer; while at the other end of the cable these same fibers are joined together, end-to-end, thereby enabling lightwaves to travel from one fiber to the other. These beams are recombined in the interferometer after traversing the same loop in opposite directions and caused to fall on a detector in an interference pattern. Variations in the resulting signal from the detector are used to detect the existence of a disturbance. However, the actual location of the disturbance is unknown.
A paper entitled "A Novel Distributed Optical Fiber Sensing System Enabling Location of Disturbance in a Sagnac Loop Interferometer," Proceedings of the SPIE, Vol. 838 at page 325 (1987) describes a combination of Mach-Zehnder and Sagnac interferometers where, along a single fiber optic path, the Mach-Zehnder interferometer has direct detection sensitivity while the Sagnac interferometer has position dependent sensitivity. By forming a ratio between the position-dependent and position-independent signals, the location and magnitude of the disturbance may be determined. Unfortunately, these different interferometers individually achieve optimum performance under different conditions which are mutually exclusive, and so a fundamental incompatibility exists between these two interferometers which is undesirable.
U.S. Pat. No. 4,976,507 discloses a Sagnac Distributed Sensor that is capable of determining the location and severity of a disturbance along a fiber optic loop. This is accomplished by operating the sensor in two distinct modes, and then processing the measurements to electronically calculate the location of the disturbance. In the first mode, the interferometer is operated in the manner described in the preceding paragraph, and a relative phase shift is generated between the counter-propagating light waves that is dependent upon both the location and the severity of the disturbance. In the second mode, the lightwave traveling in one direction around the loop is shifted in wavelength prior to traversing the loop, whereas the lightwave traveling in the other direction around the loop is shifted in wavelength after traversing the loop. Wavelength shifting is accomplished by means of a relatively expensive discrete optical frequency shifter and associated control circuitry. Less expensive intrusion locating equipment would seem possible since severity information is not desired.
What is needed and what is not yet available is an optical fiber sensing system, which is low in cost and which provides accurate information regarding the location of any intrusion that may occur within the optical communication system.